Posted
by
timothy
on Wednesday December 27, 2006 @04:11AM
from the could-often-be-my-dad's-nutty-driving dept.

An anonymous reader writes "What causes these mysterious traffic jams that continually appear throughout the day for no reason whatsoever? Is it simply the fact that most people just don't have a clue how to drive? That's very possible, and in reality there are so many variables involved in something like a traffic jam. But is it possible that the entire traffic jam could be both the continuing and end result of a chain reaction set in motion by a single driver who was in too much of a hurry?"

Quite often when it is very busy, you can see a standing wave in the traffic - there's an area where all the cars are stopped - but there is NO obstruction at all. The cars are filling the 'standing wave' from the back as quickly as cars at the front are leaving it - so it becomes self-sustaining.

When the road is full to capacity, moving at 70 mph, all it takes is one person to jab their brakes... then the drivers behind (probably following far too close) brake a bit harder, and the drivers behind them brake a bit harder still. The adjacent lanes, in seeing one lane suddenly slow go 'whoa', and someone also brakes in that lane. Pretty soon, just from one person braking a little bit - the braking has propagated down the road with greater and greater severity until one of two things happens: usually, the traffic comes to a standstill, and you get a self-sustaining standing wave of stopped traffic until the amount of traffic on the road reduces to the extent there are fewer cars joining the wave than are leaving. This can take HOURS, especially on the M6 in England. The second thing that may happen in this cascading braking severity is that someone runs into the back of the other. Then chaos ensues for most of the day.

The other problem is lorries (large trucks) overtaking lorries with a speed differential of 0.5 mph. It takes them several minutes to get past because they are both speed limited within 0.5 mph of each other, meaning the inside two lanes are 56mph, and the outside lane is 70mph+. When a frustrated driver pulls out into the outside lane after being stuck behind a lorry for "too long", they cause one of the outside lane drivers to brake down to 56 mph quite suddenly. This can easily get the 'braking cascade' started, and before you know it - you have a standing wave traffic jam with no actual obstruction (other than the standing wave itself).

Usually then what happens, is the opposite direction traffic, seeing the stoppage rubber necks for the possible accident. An inattentive driver looking at the other side of the road finally looks back in front and realises he's about to ram a truck in the rear and slams on the brakes. The driver behind him following far to closely has to brake even harder - and there's either a shunt or if they are lucky, ANOTHER standing wave traffic jam starts on this side of the road too.

It's fascinating to watch from the air. Frustrating to be in when driving.

Some of the points are very well made.Just ask anyone who has been a driver at the tail end of an Army Convoy. They are either flat out of at a dead stop. The concertina effect magnifies as the number of vehicles increases. This is why smaller convoys are better.

I was once in a lecture where this was explained. It all went down to the following
Chaos Theory
Queuing Theoryand most impostantly,
A single thing which cause on vehicle to slow down without due cause. The nthe vehicle behind has to slow and Bingo! it all starts.Once to get beyond a certain number of vehicles the elasticity in the queue gets to a critical size and you get the unexplained traffic jams.

Some places try to minimise these jams by artificially reducing speed limits to reduce the elasticity but IMHO, these have limited effect.IMHO, the ONLY way to stop these elastic jams is to connect the vehicles together. I once saw a demo of such a thing. Oh, sorry, it is called a train...:)Seriously, BMW demoed a device many years ago that would allow you to get much closer to the vehicle in front but in a safe manner. I think that it is only a metter of time before there is a viable system to connect vehicles together electronically in such a way that they can be physically very close to each other in a safe manner. The driver would join such a convoy and then switch on an autopilot system and sit back and relax.

I have lived in the cities with the worst drivers and the worst traffic and I have seen it time and time again; it's the slow dumbasses that are the real cause of majority of wrecks. It's that asshole who is going 50 in the passing lane and won't move. Or the driver is just going so slow that normal traffic rams into him, or is slowed town greatly.

The people with really fast cars generally drive very well. After all, they don't want to smash up their fancy car.

It's the assholes who don't care that they clogging up the passing lane who really are the cause of most accidents and traffic slowdown.

Oh, I have noticed that traffic patterns and behaviors do vary by location. For instance in New Orleans (pre-katrina) the drivers were extremely agressive and would not let you in no matter what and pretty much there could be aliens landing on the side of the road and nobody would care or slow down. In L.A. the 405 would be backed up forever only to find out that it was slowed down because of ONE car broke down in the emergency lane, with no accident; everyone was slowing down in response to this one car on the side of the road. In San Antonio, TX, everyone is on crack and drives a Ford F450 Dually 100mph, everywhere. Not usually a problem, but the entire city of San Antonio is being redone road wise and it creates choke points almost instantly that can't be foreseen.

It would help to cut down on the standing waves if people applied the 'join slowly, leave quickly' rule. If you see a queue ahead then slow down to give it time to move before you get there, then as soon as the road is clear accelerate away (to a safe speed). I see waves like this every day and see many people rush to join it so they have to slam on the brakes. When I get out the other side people are leaving huge gaps that slow down the escape of others.

The previous theory I heard about in a documentary about traffic in major cities, said that density waves, the same phenomenon that causes many galaxies to have perfectly defined spiral arms, also cause traffic jams, which is to say, the mathematics are the same.

As a sidenote, I once read an anecdotal story about a guy who always got stuck in the road while driving home from work, and one day he thought about how everybody's trying to get home fast yet everybody gets stuck in traffic, so he decided to experiment by driving a bit slower. After a few minutes he was amazed to find how the traffic behind him was neat and orderly, instead of the usual jumble, which implies (I emphasize: anecdotally) that the behaviour of a single car can not only create, but also avoid the creation of density waves.

See this Science Hobbyist article [amasci.com] from January 1998. It's long and detailed, and suggests practical steps individual drivers can take for breaking up (or causing!) traffic jams. Yes, dear readers, this is a nine-year-old dupe.

. . . but I doubt there's anyone who hasn't noticed that driving through any area with moderate traffic, the lights seem to work OK, but come 4-5:00pm (or 6-7:00am), irrespective of traffic flow, the lights start turning read with increasing frequency, and traffic starts backing up. My guess is there's a BOFH version of a traffic controller in every city doing it on purpose.

As I understand it, you are sort of correct. Lights are timed to slow cars down at rush hour to reduce the number of traffic accidents, resulting in a net increase in the average speed of traffic at rush hour. There is also the concept called "metering lights" which are stoplights on the freeways (bridges mostly) in the San Francisco Bay Area, The Idea is to get some space between cars so that they will move faster.

rar42 (626382) sez:> I'm inclined to compare roads to shared medium Ethernet. As the> traffic builds up you get more 'collisions' and both systems> have collision detection built-in. With Ethernet, as the 'traffic'> builds to about 40% of the theoretical capacity, collisions> become the norm

You're pretty much completely wrong, and the last quoted line sums up why.Collisions are not the norm in traffic jams.

Traffic jams happen due to the ripple effect from cumulative reaction timedelays in response to changes in traffic. The effect accumulates until thereis so much loss of speed that people drive closer together. Then when theyhave to react, they react more abruptly, and that causes yet a stronger rippleeffect.

Packets collide, cars don't. Cars change speed, packets don't.Well, OK, sometimes cars do collide. But it's not the collision itself thatcauses the traffic jam, it's the bottleneck in the right of way and/or therubberneckers.

If people could and would simply maintain the 2 second following distanceno matter what speed, when the fewer traffic jams did occur, they would resolvethemselves much more quickly. But just try telling the person 500 cars back tojust sit still for 10 minutes. They'd probably want to punch you, and they'llstill insist on driving stopgostopgostogostopgo despite the fact that doing someans they'll be doing it for several times longer than just waiting.

90% of drivers think they're better than average.90% of drivers are below average drivers.So I give free driving lessons.Like braking suddenly for tailgaters.

for example, when approaching a constriction, so many drivers go right to the end and try and force their way in - and it's not helped by people not letting adjacent lanes merge in good time before the obstruction.

Even more, it's not helped by people letting in the drivers who go to the end. Personally, I'll let people in when they try to merge soon after noticing the obstruction. But if they try to cut in at the end, I'll ride the previous car's bumper to prevent them from getting in, and give them the finger to boot! And if that makes me a bastard, that's fine with me.

But is it possible that the entire traffic jam could be both the continuing and end result of a chain reaction set in motion by a single driver who was in too much of a hurry?

No, it isn't possible that a single driver could be the cause. The mechanism described in the article relies also on a large number of other drivers all following too close to the car in front. If enough people kept a safe distance from the car in front, the shock wave caused by the sudden movement of one car would die away instead of being amplified.

Aside:There's no justice in motoring. Responsible drivers just have to get used to the idea that they can help avoid jams and accidents, and they themselves will get less benefit from their own responsible actions than will the idiots who cause the trouble in the first place. The idiots won't even realise they've done anything wrong...

I can testify that this is not the most popular reason of traffic jam, because this is not the most common disruptive behavior. Think: why the situation was happening? It is not because just one driver wanted to change the lane, this is because many drivers wanted to do that and one of them was too impatient. That takes off the load of one individual and brings it upon the condition of why the lane was slow?

Most likely the lane was slower because (a) there was a high-inertia-mass truck in front of you or (b) sloppy driver (undercaffeinated, grandma, or just plain unexperienced driver).

This condition could not be helped. The critical condition of the stand-still or bumper-to-bumper traffic jam is caused by concentration of cars increasing certain threshold level. The main factor in this criticality is the distance between cars. How many of us actually follow three-second rule? The tail-gating leads to the high probability of the scenario when the car in front of you breaks and you will be forced to break with the HIGHER deceleration. That leads to lesser control of the final steady speed achieved at the end of the process of deceleration. Needless to say that the chain reaction will continue all the way back with increasing decelaration and decreasing final speeds of deceleration.

The solution to the traffic jam problem is trying to smooth traffic even at very low speeds. To do that we need stricter laws regulating tailgating. It needs to be automatic, the cars should be equipped with automatic sensors, all the entrances to the freeways/highways should be regulated by traffic lights.

Again: the problem with traffic jam is the criticality at certain speed. The only way to lessen this criticality is to increase distance between cars.

The other good way of easing traffic jams is complete abandonment of upper speed limit. That will increase the efficency of the traffic arteries.

Together, tougher tailgating regulation and absence of speed limit, will help the traffic jam situation in the country.

The two-second gap is typically recommended for 70 MPH speeds. It is dramatically smaller for lower speeds. It is also completely unrealistic at any speed. A two-second gap at 70 MPH is about 315 feet, or about 24 car lengths. At 70 MPH the assumption is that the average person requires about 2/5th of a second to react, which equates to just under 70 feet. That leaves 247 feet for braking. The NHSTA pegs the average 70 MPH braking distance for modern cars around 170 feet. I personally suspect it's much lower these days, and with panic braking plus ABS it would be far, far lower. But give them the benefit of the doubt (though god only knows why) and call it 160 feet. That leaves you 87 feet, so the two-second gap is overly conservative by a factor of about 25%.

BUT -- and here is where it gets stupid for real-world conditions -- that braking scenario assumes that you must stop within that two-second gap. Think about this: the only way that would matter is if there is an immobile object two seconds ahead of you. You're driving along, then mysteriously, 315 feet in front of you, something is stopped dead. What are the actual chances of this happening to any responsible, alert driver doing 70 MPH? Very small. In fact what will happen is that the car ahead begins to slow, and you burn your 0.4 second reaction time (which I also think is unrealistically high), then you begin to slow in concert with the car ahead. It is obviously impossible to derive any specific numbers for the rates at which this happens as they'll be random, but it certainly doesn't equate to a 70MPH-to-zero panic stop in a limited space.

In any case, his figure of 120 cars per minute is probably a lot closer to reality than anything provided for by the 2-second rule, which is a 24-car-length gap -- have you ever seen a busy highway where anyone was maintaining a 24-car-length gap? Would it even be possible to actually estimate and maintain this at 70 MPH? That's about 1/16th of a mile... 16 cars per mile and the highway would look positively empty.

...light rail is working in Los Angeles. What's the fastest way to get Downtown? The Red Line. 30 minutes from NoHo to Union Station. Un-freaking-believable.

This is why when the price of gas went up, and people actually tried the Red Line and Metrolink and other parts of our old/new (most of the right-of-ways are old Pacific Electric right-of-ways) light rail infrastructure, people started talking about how nice it would be to have the Wilshire spur of the Red Line finally take its intended trip to Santa Monica. The Expo Line between USC and Culver City (with an extension to Venice on the drawing board) is being built now. It links with the oldest of our light rail lines, the Blue Line, which goes down the Alameda corridor through some of the nastiest neighborhoods in LA. And yet: the Blue Line gets a lot of use. Why? It's the easiest way to get to Long Beach.

We have our ill-advised lines too: the Green Line which boneheadedly does not go all the way to LAX, and the Gold Line which is a good route into Pasadena from Downtown but is slowed to a crawl by nervous NIMBYs who don't have the good sense to tell their kids to GIVE RAILROAD RIGHT-OF-WAYS A WIDE BERTH. "Oh, those Blue Line trains go so fast and we see lots of people killed on the 6:00 news...oh please, City Councilperson, make 'em run the trains at 15 miles per hour in our neighborhood!" Dumbasses. You can't outrun a train, either on foot or in your freaking car. Grow a freaking brain.

We need light rail. It made sense when they put the Pacific Electric in back in the day, and it makes even more sense now. With Peak Oil on the horizon it's time. When gasoline hits $5 a gallon maybe even the NIMBYs will have a nice warm cup of STFU and get on board.

Sometimes you can tell the difference. On my commute home every day there is a one-lane offramp with a stop light at the top, and it gets backed up a half mile or so. As I approach the final merge point, most drivers who are still trying to get in have arrived at full cruising speed...an obvious attempt to "cut in line." I will ride the bumper of the car in front of me like I was on his trailer hitch to keep these jerks out. Other times, usually much further back, drivers are trying to get in late but have obviously made a good faith effort...I let these in when able.

And that reminds me of a major gripe I have. In some places, usually where there is an offramp and an onramp that share a lane for a tenth of a mile or so (ie, they cross,) if there is standing traffic in the travel lanes I will encounter some drivers who won't let me merge from the onramp. I mean, where the hell am I supposed to go? It's not like I'm trying to cheat anyone out of their spot or anything! My lane disappears into your lane...what do you want me to do? Idiots. I can only imagine that these people have very small worlds, and take it as some sort of defeat that a car was able to get in front of them.

In our area, there is a twice-daily traffic jam that has been understood for years, but fixing the road to take away the problem would be ungodly expensive.

There is, actually, nothing technically wrong with the road. The road in question is I-87 (the Northway), and the pinch point is where it crosses the Mohawk river. The Twin Bridges have a slightly narrower shoulder than the highway leading up to them in either direction, but the shoulder, on both sides of each bridge, is still every bit as wide as any of the three lanes going in either direction.

Compounding the problem is that the bridges are (hope this is the right term) truss bridges. There are two convex bowed beams that go over each side of each bridge, and a construct of triangular trusses between them. These are the reason why a change would be ungodly expensive, because you would have to rebuild the bridges.

Anyway, people come to the bridges and slow down because they perceive that the road has gotten narrower, while failing to perceive that this fact is irrelevant. This slowing down leads to the accordion effect that was described in TFA, where successive cars have to apply more and more braking in order not to hit the car in front of them. By the time you are a mile north of the bridge in the mornings (south in evenings), traffic is basically stopped.

The construct that causes all of this trouble can be seen here [google.com] (along with some Google wierdness in the construction of the image).

Good (if obvious) point. A traffic jam can be caused by too many cars, by stupid drivers, or by both. For example, if a high capacity road has to "dump its load" onto an intersection with a traffic light, the traffic will still back up, regardless of the mental abilities of the drivers. On the other hand, I've seen traffic back up, even when the freeway is well below capacity, simply because of the right combination of stupid drivers. Such as when the two people in the middle and outer lanes decide to drive the same speed, creating an impassible wall, and the driver in the inner lane decides it's his GOD-GIVEN INALIENABLE RIGHT to drive whatever speed he damn well pleases in that lane, even if he's just going to camp there.

Another thing people don't seem to realize is that "more roads" or "wider roads" won't usually constitute a long-term solution to traffic problems. It just leads more people to plan around the roads having higher capacity, so more and more people plan on using the roads, until they're choked up again. See my last journal entry.

So, why don't they just move the "narrowing" of the shoulder upfield? That is _gradually_ introduce the reduced width before the approach to the bridge, in fact why not make the approach artificially narrower before the bridge and make it appear the bridge is wider than the approach, at the exit off the bridge keep it as is (percieved widening) then do the same for the opposite direction.

but it certainly doesn't equate to a 70MPH-to-zero panic stop in a limited space.

You have obviously never driven on 880 through the East Bay between 6am and 8pm.

In my opinion you can frequently blame random mass panic stops on on guy who's already home snacking and watching the tube, because the guy to blame is an idiot traffic engineer that works for the state, and they get to go home at 3:30pm.

Seriously..

In all the years I've driven I've decided that traffic flow is analagous to a special case of internal compressible flow. That is the mechanical engineering way to say it's like supersonic air flow in a pipe. The special case is that there is no friction with the walls of the pipe and that the pipe is very small in diameter. When traffic is at a sufficient density and speed, a constriction, or obsruction can cause a shock to occur. On one side of the shock traffic is "supersonic," on the other side it's crawling along, and at the shock everyone is standing on their brakes hoping they won't get rear ended by that idtiot in the SUV with the cell phone and starbucks. Shocks can be static. This tends to occur when there is no change in total mass flow rate across the shock. A moving shock occurs when there is a change in mass flow across the shock. If total trafic flow rate is slower downstream of a shock then the shock will move backward through traffic until it reaches an equilibrium point. Sometimes you get a bad phenomena that I call cascading shocks. That's when some constriction causes a shock for just a moment. That shock, if the traffic is of sufficient density and speed, travels backwards through traffic, but since the source is temporary, or periodic, traffic accelerates again (since gas flows don't accelerate on thier own, that is where the analogy breaks down). Then the disturbance occurs again and another shock starts moving backwards through traffic. One example of this that I can think of is on 880 south approaching the San Mateo Bridge turn off. 880 tends to be very fast and very high density. There is a carpool lane. Frequently a jerk waits as long as possible in the carpool lane before frantically changing across all the lanes to exit to the Bridge. This causes a moving shock, and so does the next jerk... This is why on that freeway everybody will be doing 70 one moment and the next everyone is standing on their brakes, hoping the sphincter will hold. A moment later everyone is going 70 again, and sure enough here comes another panic stop.

So clearly in flow that has a critcal density and speed, any sufficiently large perterbation can cause a traffic slowdown. Chaotic effects clearly come into play.

A better solution is to strictly enforce following distance laws. A study was done on traffic patterns in Seattle area to see how they could be improved and found that the single most important thing to fix the issue was to give proper following distance between cars. This counteracts the pipeline bubbles (yes almost the same thing as a bubble in your processor pipeline) by giving room to absorb sudden stops and also solves some of the merging issues that we see. Its more likely than not that even though the one jackass that was spoken about in the summary triggered the slowdown the real cause was everyone crowding the person in front of them so that the sudden stop was propogated in perpetuity instead of being absorbed and spread across the whole flow.

Modern cars will give much better braking control under many more conditions. But under ideal conditions you'll find that stopping distances have hardly changed at all in 40 years. The limit of braking power is still the coefficient of friction between rubber and asphalt, and no-one is likely to break the laws of physics anytime soon.

"90% of drivers think they're better than average.90% of drivers are below average drivers.So I give free driving lessons.Like braking suddenly for tailgaters."

For irresponsible people like you who play fast and loose with the lives of other people on the road I have a favorite past time. I like to call it "Civic Lesson 101" or YANA (Your Ass is Not Anonomyous.)

What many people fail to recall (or do not know in the first place) is that their license plate number is linked to the public record of their automobile registration. Just as a police officer can access your records from his cruiser by entering your license plate into his computer, so I can access your public record from any PC with an internet connection.

Your public record contains useful data on how I can get in touch with you such as: your address, your full name, when and where you bought your vehicle, and what finance company is financing it. Sometimes it even has your phone number listed; however this is a convenience as a few moments with Google is just as effective. Someone motivated by sinister desires, vengeance, or worst of all a sense of justice could wreak incredible personal, financial, and even legal harm with these details. For me the mere inconvenience and/or uneasiness of the perp is enough. All I want is to remind them...YANA: act appropriately.

At this point the options are wide open. If the trangression was eggregious enough (road rage or say blocking the left lane on a multilane highway while driving under the speed limit and then aggressively braking) there can be severe penalties. In highschool we would grab a few friends, scrape together $40, buy 4000 forks, and then carefully stick them into the front yard of the offending idiot. Others have had fun with rock salt on the grass, but I draw the line at destruction of property.

As an adult I use a more civilized approach. Simply send an anonomyous letter addressed to the family of the perpetrator detailing the incident in question and how you attained their home address. Polite reminders about what someone of a more violent disposition might do under these circumstances are unnecessary. The imagination of the recipient of the letter is sufficient in this respect.

A follow-up phone call during business hours may or may not be necessary, though it is undeniably effective. The idea is to sound as a voice of reason, acting in the interests of public safety. Always be polite, professional, and as unemotional as Hal9000. Briefly describe the incident and ask the perpetrator (or better their spouse) to stop driving in that manner. You only make the phonce call AFTER they receive the letter, otherwise they tend to get hostile and quite rude. The letter tends to bring people down a notch as they realize that they are exposed while you, the slighted party, remain anonomyous.

Whether or not this has any lasting effect is unknown, however I think it does make people a little more aware that the vehicles they toy with on the road actually contain human beings...and remember that humans are unpredictable and...YANA.